R.W. Chantrell

Switching mechanisms in cobalt-phosphorus thin films

Studies have shown that although the interactions between grains in magnetic thin-film recording media can enhance the squareness, these interactions are also responsible for media noise. The authors made measurements on Co-P thin films that give a simple and practical way of investigating the role of interactions in the magnetic processes that occur in these films. All magnetic measurements were made on a PAR 155 vibrating sample magnetometer with a resolution of 10/sup -5/ EMU. The field measurements were made with a Bell-Gauss Hall probe calibrated with a proton magnetometer to give an accuracy of >

Transient ferromagnetic-like state mediating ultrafast reversal of antiferromagnetically coupled spins

Ferromagnetic or antiferromagnetic spin ordering is governed by the exchange interaction, the strongest force in magnetism. Understanding spin dynamics in magnetic materials is an issue of crucial importance for progress in information processing and recording technology. Usually the dynamics are studied by observing the collective response of exchange-coupled spins, that is, spin resonances, after an external perturbation by a pulse of magnetic field, current or light. The periods of the corresponding resonances range from one nanosecond for ferromagnets down to one picosecond for antiferromagnets. However, virtually nothing is known about the behaviour of spins in a magnetic material after being excited on a timescale faster than that corresponding to the exchange interaction (10–100 fs), that is, in a non-adiabatic way. Here we use the element-specific technique X-ray magnetic circular dichroism to study spin reversal in GdFeCo that is optically excited on a timescale pertinent to the characteristic time of the exchange interaction between Gd and Fe spins. We unexpectedly find that the ultrafast spin reversal in this material, where spins are coupled antiferromagnetically, occurs by way of a transient ferromagnetic-like state. Following the optical excitation, the net magnetizations of the Gd and Fe sublattices rapidly collapse, switch their direction and rebuild their net magnetic moments at substantially different timescales; the net magnetic moment of the Gd sublattice is found to reverse within 1.5 picoseconds, which is substantially slower than the Fe reversal time of 300 femtoseconds. Consequently, a transient state characterized by a temporary parallel alignment of the net Gd and Fe moments emerges, despite their ground-state antiferromagnetic coupling. These surprising observations, supported by atomistic simulations, provide a concept for the possibility of manipulating magnetic order on the timescale of the exchange interaction.

Ultrafast heating as a sufficient stimulus for magnetization reversal in a ferrimagnet

The question of how, and how fast, magnetization can be reversed is a topic of great practical interest for the manipulation and storage of magnetic information. It is generally accepted that magnetization reversal should be driven by a stimulus represented by time-non-invariant vectors such as a magnetic field, spin-polarized electric current, or cross-product of two oscillating electric fields. However, until now it has been generally assumed that heating alone, not represented as a vector at all, cannot result in a deterministic reversal of magnetization, although it may assist this process. Here we show numerically and demonstrate experimentally a novel mechanism of deterministic magnetization reversal in a ferrimagnet driven by an ultrafast heating of the medium resulting from the absorption of a sub-picosecond laser pulse without the presence of a magnetic field.

Agglomerate formation in a magnetic fluid

Monte Carlo techniques have been used to investigate the effects of magnetostatic and repulsive particle interactions on the formation of agglomerates in a magnetic fluid. The dependence on particle size and applied field of the form of the agglomerates was studied using a spatial distribution function which allows a quantitative distinction to be made between clusters and anisotropic chain structures. Magnetization curves have been calculated for magnetic particle sizes varying from 5 to 15 nm with and without magnetostatic interactions. For the larger particle sizes, it was found that the initial susceptibility is reduced in the presence of interactions. This is associated with the presence of pronounced agglomeration in zero field, where open clusters are formed. As the applied field is increased the clusters break up to form long chains aligned in the field direction. At intermediate particle sizes, there is evidence of magnetic field induced agglomeration leading to the formation of dimers and trimers preferentially aligned in the field direction. The smallest particle size showed little evidence of ordering even in strong applied fields, since thermal disordering dominates the situation.

Spin-glass behavior in a fine particle system

A study of the magnetic behavior characteristic of a spin-glass, which has been examined for systems of ultrafine particles, is reported. The results obtained support the suggestion of E. P. Wohlfarth (1977) that spin-glass effects could arise from the clustering of impurity spins in the host material into ferromagnetic rich clusters. All the results are explained on the basis of the Neel superparamagnetic (blocking) model. The results obtained show that the behavior of fine particle systems is similar to that of certain spin-glass alloys (e.g., Au/Fe alloys). This result supports the idea that spin-glass alloys can be interpreted on the basis of a blocking model. >

Temperature-dependent magnetic properties of FePt : Effective spin Hamiltonian model

A model of magnetic interactions in the ordered ferromagnetic FePt is proposed on the basis of first-principles calculations of non-collinear magnetic configurations and shown to be capable of explaining recent measurements of magnetic-anisotropy energy (MAE). The site (Fe,Pt) resolved contributions to the MAE have been distinguished with small Fe easy-plane and large Pt easy-axis terms. This model has been tested against available experimental data on the temperature dependence of MAE showing scaling of uniaxial MAE (K1(T)) with magnetization (M(T)) K1(T) ~ M(T)γ characterized by the unusual exponent of γ = 2.1. It is shown that this unusual behavior of the FePt can be quantitatively explained within the proposed model and originates from an effective anisotropic exchange mediated by the induced Pt moment. The latter is expected to be a common feature of 3d-5d(4d) alloys having 5d/4d elements with large spin-orbit coupling and exchange-enhanced Stoner susceptibility.

Susceptibility phenomena in a fine particle system: I. Concentration dependence of the peak

Abstract The concentration dependence of the peak in the dc susceptibility of a weakly interacting fine particle system consisting of Fe 3 O 4 particle has been studied. Measurements of the initial susceptibility of the dispersion in the solid state show that the susceptibility values were depressed at low temperatures. The temperature of the peak ( T g ) in the susceptibility versus temperature curve was found to occur at higher temperatures as the concentration was increased. The variation of T g with concentration shows a simple power law relationship. The influence of dipolar interactions between the particles on the value of T g is discussed by considering the effects of particle interactions on the blocking temperature. In this study we develop an effective blocking temperature model, and apply it to our system.

A model of interaction effects in granular magnetic solids

The effects of interactions (dipolar and exchange) on the magnetic behavior of granular solid systems are examined using a Monte Carlo model capable of predicting the temperature and time dependence of the magnetic properties. Using this model the interaction effects on the magnetization and the magnetoresistance are studied. The results show that these properties depend critically on the strength and nature of the interactions. Magnetostatic interactions are found to decrease both remanence and coercivity and Hc is predicted to decrease linearly with concentration. It is shown that spatial disorder may be responsible for an increase of coercivity with exchange coupling which has been observed in some experimental studies. In systems with no hysteresis, magnetostatic interaction effects are found to increase the superparamagnetic transition temperature, in agreement with experimental data and previous analytical treatments. Calculations of the giant magnetoresistance (GMR) show that magnetostatic interact...

Static and dynamic experimental studies of particulate recording media

Abstract Measurements are presented of the static and dynamic magnetic properties of magnetic recording tapes and powders, with the aim of investigating the effects of particle, particle interactions. A detailed study was made of the principal remanence curves, a comparison of which was found to highlight the many-body effects. This phenomenon is discussed in relation to similar effects in hard magnetic materials. It was also found that an ac field demagnetised samples significantly faster than the equivalent dc field, contrary to the predictions of a dynamic model of non-interacting particles.

Structural studies of L10 FePt nanoparticles

We have studied the lattice parameter changes of L10 FePt nanoparticles annealed to near equilibrium as a function of composition by x-ray diffraction. We have found that the (111) diffraction peak shifts linearly with composition, however, the c parameter mostly changes in the Pt rich compositions and the a parameter mostly changes in the Fe rich compositions with respect to the equiatomic composition. This causes the tetragonality of the L10 structure to be maximized near the Fe 50%/Pt 50% composition. The magnetic properties were measured at room temperature and at 5 K and are correlated to the structural changes occurring as a function of composition.